<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
</head>
<body>
<canvas id="webgl" width="500" height="500" style="background-color: #0d72da"></canvas>
<!-- 顶点着色器源码 -->
<script id="vertexShader" type="x-shader/x-vertex">
  //attribute声明vec4类型变量apos
  attribute vec4 apos;
  void main() {
    //设置几何体轴旋转角度为30度，并把角度值转化为弧度值
    float radian = radians(30.0);
    //求解旋转角度余弦值
    float cos = cos(radian);
    //求解旋转角度正弦值
    float sin = sin(radian);
    //引用上面的计算数据，创建绕x轴旋转矩阵
    // 1      0       0    0
    // 0   cosα   sinα   0
    // 0  -sinα   cosα   0
    // 0      0        0   1
    mat4 mx = mat4(1,0,0,0,  0,cos,-sin,0,  0,sin,cos,0,  0,0,0,1);
    //引用上面的计算数据，创建绕y轴旋转矩阵
    // cosβ   0   sinβ    0
    //     0   1   0        0
    //-sinβ   0   cosβ    0
    //     0   0   0        1
    mat4 my = mat4(cos,0,-sin,0,  0,1,0,0,  sin,0,cos,0,  0,0,0,1);
    //两个旋转矩阵、顶点齐次坐标连乘
    gl_Position = mx*my*apos;
  }

</script>
<!-- 片元着色器源码 -->
<script id="fragmentShader" type="x-shader/x-fragment">

  void main() {
    // 逐片元处理数据，所有片元(像素)设置为红色
    gl_FragColor = vec4(1.0,0.0,0.0,1.0);
  }

</script>

<script>
    //通过getElementById()方法获取canvas画布
    var canvas=document.getElementById('webgl');
    //通过方法getContext()获取WebGL上下文
    var gl=canvas.getContext('webgl');

    //顶点着色器源码
    var vertexShaderSource = document.getElementById( 'vertexShader' ).innerText;

    //片元着色器源码
    var fragShaderSource = document.getElementById( 'fragmentShader' ).innerText;
    //初始化着色器
    var program = initShader(gl,vertexShaderSource,fragShaderSource);
    //获取顶点着色器的位置变量apos
    var aposLocation = gl.getAttribLocation(program,'apos');

//        8个顶点坐标数组
    var data=new Float32Array([
        0.5,  0.5,  0.5,//顶点0
        -0.5,  0.5,  0.5,//顶点1
        -0.5, -0.5,  0.5,//顶点2
        0.5, -0.5,  0.5,//顶点3
        0.5,  0.5, -0.5,//顶点4
        -0.5,  0.5, -0.5,//顶点5
        -0.5, -0.5, -0.5,//顶点6
        0.5, -0.5, -0.5,//顶点7
    ]);
//        顶点索引数组
    var indexes = new Uint8Array([
//        前四个点
        0,1,2,3,
//        后四个顶点
        4,5,6,7,
//        前后对应点
        0,4,
        1,5,
        2,6,
        3,7
    ]);

    //创建缓冲区对象
    var indexesBuffer=gl.createBuffer();
    //绑定缓冲区对象
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER,indexesBuffer);
    //索引数组indexes数据传入缓冲区
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER,indexes,gl.STATIC_DRAW);

    //创建缓冲区对象
    var buffer=gl.createBuffer();
    //绑定缓冲区对象
    gl.bindBuffer(gl.ARRAY_BUFFER,buffer);
    //顶点数组data数据传入缓冲区
    gl.bufferData(gl.ARRAY_BUFFER,data,gl.STATIC_DRAW);
    //缓冲区中的数据按照一定的规律传递给位置变量apos
    gl.vertexAttribPointer(aposLocation,3,gl.FLOAT,false,0,0);
    //允许数据传递
    gl.enableVertexAttribArray(aposLocation);


    //LINE_LOOP模式绘制前四个点
    gl.drawElements(gl.LINE_LOOP,4,gl.UNSIGNED_BYTE,0);
    //LINE_LOOP模式从第五个点开始绘制四个点
    gl.drawElements(gl.LINE_LOOP,4,gl.UNSIGNED_BYTE,4);
    //LINES模式绘制后8个点
    gl.drawElements(gl.LINES, 8, gl.UNSIGNED_BYTE, 8);


    //声明初始化着色器函数
    function initShader(gl,vertexShaderSource,fragmentShaderSource){
        var vertexShader = gl.createShader(gl.VERTEX_SHADER);
        var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
        gl.shaderSource(vertexShader,vertexShaderSource);
        gl.shaderSource(fragmentShader,fragmentShaderSource);
        gl.compileShader(vertexShader);
        gl.compileShader(fragmentShader);
        var program = gl.createProgram();
        gl.attachShader(program,vertexShader);
        gl.attachShader(program,fragmentShader);
        gl.linkProgram(program);
        gl.useProgram(program);
        return program;
    }
</script>
</body>
</html>
